drm/i915: Remove unused i915_gem_active_peek_rcu()

drm/i915: Remove unused i915_gem_active_peek_rcu()

This was originally introduced to be used by the busy-ioctl, but in the
end busy ioctl performed a different dance. Since there are no users,
and no likely users, remove an unwanted chunk of the API.

Suggested-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Cc: Daniel Vetter <daniel.vetter@ffwll.ch>
Link: http://patchwork.freedesktop.org/patch/msgid/1470728222-10243-1-git-send-email-chris@chris-wilson.co.uk
Reviewed-by: Daniel Vetter <daniel.vetter@ffwll.ch>